Abstract

The feasibility of limiting greenhouse gas concentrations and associated global mean temperature increase to 2 degrees C above preindustrial levels has recently attracted considerable scientific and policy attention. Whether or not such low targets can be achieved in the long-term depends on a number of assumptions about, for instance, technological change and the willingness of countries to immediately join a post-Kyoto agreement to limit anthropogenic climate change. As part of the EMF22 international scenarios study, our paper contributes to the ongoing discussion by conducting a scenario analysis in order to examine the influence of these assumptions on the feasibility of a set of radiative forcing targets. We distinguish between "first-best-scenarios" with full participation, and "second-best-scenarios" with delayed participation of Brazil, China, India, and Russia in 2030 and the rest of Non-Annex I countries after 2050. In addition, we explored the robustness of our "first-best" scenario results by varying the assumptions with regard to technological availability of key technology clusters. Accounting for the deep uncertainties associated with future global mean temperature change, we conduct a probabilistic assessment and explore the scenario's likelihood for staying below a range of different temperature thresholds.

Our results indicate the need for a diverse portfolio of mitigation technologies in order to decarbonize the energy system. Efficiency and enhanced energy conservation play an important role across all scenarios. In addition, carbon capture and storage (CCS) is found to be important for achieving particularly low forcing targets as well as for avoiding pronounced overshoot of the target in the medium-term. Negative emission technologies such as bioenergy with CCS have a strong influence on the timing of mitigation; i.e. without their availability much more early action is needed.

According to our analysis, significant delay in the participation of major emitters leads to the unattainability of the most stringent forcing targets. Moreover, delayed participation may lead to excessive additional global mitigation cost that may even exceed the mitigation costs of significantly most stringent forcing targets in the best case with full participation. We find that for countries that significantly delay their participation, the early gains from postponing mitigation may be offset to a large extent in the long-term, due to the need for more pronounced mitigation later in the century. Linking the radiative forcing targets with temperature change, we find that the chances of staying below certain temperature thresholds may be significantly lower over the course of the century than would be by looking exclusively at 2100. From a policy perspective our results therefore suggest that focusing on early abatement to avoid significant overshoot of concentrations may be as important as the long-term concentration target itself.